The interferometric examination of the form of rotational symmetrical, aspherical lens surfaces or mirror surfaces is conventionally carried out in that a test wave usually having a spherical waveform is reflected at the component under test. Thereafter, the difference of the optical light path length with respect to a known reference surface is determined for the different points of incidence. This reference surface is usually arranged in a reference arm of the interferometer whereby interference patterns are produced with coherent light. The actual form of the surface can be determined when these interference patterns are evaluated quantitatively.
The process described above is only possible for surfaces which are only slightly aspherical and for which the deviation from a sphere corresponding closely thereto is only very slight. In contrast to these aspherical surfaces, the deviation between the spherical test wave and the aspherical surface to be tested is as a rule so great that the above-described method can no longer be applied in this simple form. The test rays impinge perpendicularly in only a narrow zone on the aspherical surface to be tested, namely, at that location where the spherical test wave touches the asphere. Beyond this zone, the deviations between the directions of the incident and reflecting rays become ever greater. Ultimately, the reflected rays are no longer received by the downstream optic and are completely lost for the interference pattern to be generated. However, rays which nonetheless pass through all optics and diaphragms exhibit considerable light path length differences of many wavelengths and generate fringe distortions in the interferogram which are so great that an evaluation of the interferogram is no longer possible.
For the above reasons, special optical lens systems known as so-called compensation systems are placed in the beam path of the test wave for testing aspherical surfaces. These compensation systems coact with the desired asphere to be tested to again provide a stigmatic wave. The compensation systems therefore adapt the wavefront of the test wave to the form of the asphere to be tested. A defective test component then generates only small light path length differences to the reference wave. The interference fringes are therefore only slightly distorted and the interferogram can thereby be evaluated quantitatively.
However, these methods only function accurately when the optical effect of the manufactured compensation system is precisely known since this system is included in the test result. It is not possible to examine the compensation system in and of itself. For this purpose, a master aspherical component must be made available which, in turn, cannot be precisely tested.
The compensation systems must therefore be manufactured with the greatest of care and highest obtainable precision. All parameters which get included in the optical result must be precisely maintained and individually measured. These parameters include the index of refraction of the glass material used, the homogeneity of the glass material, the lens radii, lens thicknesses, and air distances as well as the adaptation of the lenses to each other. The frame must guarantee the precise centering. However, even with this considerable effort, no defect-free compensation system can be produced.
In order to avoid the above-mentioned problem, the suggestion has already been made that the compensation system be replaced with synthetic holograms. Such a method is, for example, disclosed in U.S. Pat. No. 4,396,289. These holograms are especially computed for adaptation to the asphere to be tested and can be plotted on a suitable carrier. However, as a rule, synthetic holograms alone are not adequate to adapt the wavefront to the component to be tested. The holograms must therefore often be combined with lens compensation systems of simple configuration. However, this again is associated with an adjustment. The correct masking out of unwanted diffraction orders present further difficulties. Such masking out can require a so-called "off-axis" angle for establishing the hologram. Other difficulties are the low diffraction efficiency in the holograms and the wavefront errors caused by the hologram carrier itself. For these reasons, although computer holograms can be used in laboratories, they can hardly be utilized in production facilities where the simplest possible configuration is wanted.
It is also already known to record the interferograms with a camera and to evaluate the interferograms with a computer coupled to the camera. The aspheres can then be illustrated in the same mathematical form in which they were formulated. With interferometers of this kind, it is possible to do without a complete compensation of the wavefront of the test wave. Rather, a simple compensation optic is used and a part of the measuring range of the interferometer is used to measure the remaining residual of the asphericality which the compensation optic leaves and to eliminate the same from the test result by computation. However, this method has limits since the measuring range which can be evaluated at large deviations between the test aspherical component and the wavefront of the test wave is quickly reached.
It has also been suggested to test aspherical components interferometrically by sections. For this purpose, ring-shaped component regions of the aspherical component are sequentially interferometrically measured with these component regions having a form corresponding to the wavefront of the testing wave adapted to these component regions. A method of this kind is disclosed, for example, in U.S. Pat. No. 4,743,117. In this method, a test wave with a ring-shaped aperture is directed onto the surface of the asphere to be tested. Displacing the asphere assures that the test wave is again reflected approximately in autocollimation and thereafter, the reflected annular bundle of rays is evaluated with the aid of a Shearing interferometer.
However, no reliable connection of the tested component regions to each other is obtainable. Instead, the measurement errors are additive over the sequential connection of tested component regions so that the total form of the asphere can be determined only with a relatively large measurement uncertainty.
A test arrangement for aspheres on the basis of a so-called Fizeau interferometer is described in an article entitled "Rotating Scan Interferometer" by P. Langenbeck appearing in the journal "Proceedings of the SPIE", Volume 396 (1983), pages 99 to 101. In this method, the Fizeau objective is displaceable relative to the test component. Here too, ring-shaped component regions of the aspheres are examined sequentially with respect to manufacturing nonuniformities More specifically, it is those annular zones which are examined wherein the osculating radius of the asphere corresponds to the spacing of the focus point of the spherical reference wave generated by the Fizeau objective.
In this arrangement too, the tested component regions cannot easily be connected to each other in such a manner that the form of the entire asphere results therefrom. This condition is present because the relative position between the collimator objective and the test component is not measured so that no computated connection of the component regions to each other is possible. Furthermore, the optical light path length difference in the test arm of the interferometer changes for the total test surface because of the displacement of the Fizeau objective whereby the correspondence of the individual interference orders to one another in the different positions of the objective is lost.
U.S. Pat. No. 4,074,937 teaches that an axially displaceable collimator optic can be provided in a Fizeau interferometer between the reference surface of the Fizeau objective and the test component for examining surfaces having relatively large radii. The displacement of this collimator optic adapts the radius of the test wavefront to test components with different radii. A measurement of aspherical test components is not described in this patent.
U.S. Pat. No. 4,387,994 describes a Twyman-Green interferometer wherein an aspherical test component in the test arm of this interferometer is measured against a reference object mounted in the reference arm of the interferometer. The interferograms are here evaluated electronically with a CCD-camera. Either the test object or the reference object is axially displaced and the points of maximum contrast for the different positions are statistically evaluated. It is especially disadvantageous in this method that a corresponding reference object is required for each asphere to be tested.